Yen-Lin Han

Experimental and Computational Studies of Temperature Gradient–Driven Molecular Transport in Gas Flows through Nano/Microscale Channels

Studies at the University of Southern California have shown that an unconventional solid-state device, the Knudsen compressor, can be operated as a microscale pump or compressor. The critical components of Knudsen compressors are gas transport membranes, which can be formed from porous materials or densely packed parallel arrays of channels. An applied temperature gradient across a transport membrane creates a thermal creep pumping action. Experimental and computational techniques that have been developed for the investigations will be discussed. Experimental studies of membranes formed from machined aerogels, activated by radiant heating, have been used to investigate thermal creep flows. In computational studies, several approaches have been employed: the direct simulation Monte Carlo (DSMC) method and discrete ordinate solutions of the ellipsoidal statistical (ES) and Bhatnagar-Gross-Krook (BGK) kinetic models. Beyond the study of Knudsen compressor performance, techniques discussed in this article could be used to characterize the properties of gas flows in nano/microscale channels.

Knudsen Compressor Performance at Low Pressures

The Knudsen Compressor is a solid‐state micro/meso‐scale gas roughing pump based on the rarefied gas phenomena of thermal transpiration. Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membrane, typically a silicon aerogel membrane. Previous optimization studies have concluded that significant reductions of both energy consumption and device volume per unit throughput and pressure difference can be achieved when each stage of the cascade operates with a Knudsen Number based on the mean pore radius of approximately one. Perforated aerogels (using the same bulk aerogel material, but with machined arrays of properly sized parallel capillaries) are appealing candidate low‐pressure transpiration membranes and are the focus of this investigation. Conventional drilling techniques using micro drills have successfully demonstrated perforated aerogel with pore diameters ranging from 210μm to 1mm. This range of pore sizes corresponds to efficient Knudsen Compressor operation between roughly 1 Torr and 10 mTorr. The other issue at low pressures is the larger Kn of the connector section which can introduce “reverse” thermal transpiration. Two conventionally perforated carbon doped aerogel membranes, with the mean pore diameters of 210 μm and 380 μm, have been tested at the operating pressure range of 2 Torr to 10 mTorr. Comparison with the predicated results showed the evidence of rarefied gas effects such as “reverse” thermal transpiration.The Knudsen Compressor is a solid‐state micro/meso‐scale gas roughing pump based on the rarefied gas phenomena of thermal transpiration. Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membrane, typically a silicon aerogel membrane. Previous optimization studies have concluded that significant reductions of both energy consumption and device volume per unit throughput and pressure difference can be achieved when each stage of the cascade operates with a Knudsen Number based on the mean pore radius of approximately one. Perforated aerogels (using the same bulk aerogel material, but with machined arrays of properly sized parallel capillaries) are appealing candidate low‐pressure transpiration membranes and are the focus of this investigation. Conventional drilling techniques using micro drills have successfully demonstrated perforated aerogel with pore diameters ranging from 210μm to 1mm. This range of pore sizes corresponds to ef...

Dimensional and Structural Control of Silica Aerogel Membranes for Miniaturized Motionless Gas Pumps

With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol-gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump applications. The engineering of such aerogel membranes, which implies pore structure control and chemical surface modification, requires both chemical processing know-how and a detailed understanding of the influence of the material properties on the spatial flow rate density. Optimal pumping performance was found for devices with integrated membranes with a density of 0.062 g cm(-3) and an average pore size of 142.0 nm. Benchmarking of such low-density hydrophobic active aerogel membranes gave an air flow rate density of 3.85 sccm·cm(-2) at an operating temperature of 400 °C. Such a silica aerogel membrane based system has shown more than 50% higher pumping performance when compared to conventional transpiration pump membrane materials as well as the ability to withstand higher operating temperatures (up to 440 °C). This study highlights new perspectives for the development of miniaturized thermal transpiration air pumps while offering insights into the fundamentals of molecular pumping in three-dimensional open-mesoporous materials.

Characterization of a Radiantly Driven Multistage Knudsen Compressor

Knudsen Compressors are meso/micro scale gas compressors/pumps based on thermal transpiration or thermal creep. The design of radiantly driven Knudsen Compressors is discussed, along with a model that was developed to understand their performance. Experimental pumping performances for Knudsen Compressors with one, two, five, and fifteen stage, radiantly driven cascades are also discussed. Temperature measurements across the transpiration membranes, for various pressures of Nitrogen, were obtained and compared to those predicted by the performance model. The results agree with the model to within 15% consistently under predicting the measured hot side temperature of the transpiration membrane. The pump-down curves, steady-state maximum pressure differences, and maximum flow rates produced by a single stage Knudsen Compressor were obtained. A variety of configurations were studied at pressures from 500 mTorr to atmospheric pressure. The experimental results agreed with the performance model’s predictions to within 20%.Copyright

CONTINUOUS LOW POWER PRE-CONCENTRATIONS FOR DISTRIBUTED MICROSCALE TRACE GAS ANALYSIS

Continuously operating pre-concentrators for mobile and distributed trace gas analyzers have been investigated. Micro/meso-scale trace gas concentrations increasing by a factor of 10 per stage appear to be realistic. At sampling flow rates of a few tenths of a mL/s the pumping power costs can be as low as 50 mW. Separation membranes based on quantum filtering, size filtering and diffusive mass filtering are compared. Dual function size and diffusive mass filtering membranes are the most attractive possibility. Candidate pumping systems are discussed. Concentration cascades are possible and an illustrative three stage cascade is discussed.Copyright

Performance analysis of the continuous trace gas preconcentrator

In gas molecule detection systems, certain trace gas components can go undetected. This is due to ultralow yet dangerous concentrations combined with limitations of the detection methods. To remedy this problem, a preconcentrator can be included in a system to increase the trace gas concentrations, before the gas samples enter the detection unit. The widely used adsorption/desorption preconcentrators enable detection by interrupting the sampled gas flow for significant periods, in order to accumulate detectable periodic concentrations of trace gas molecules. The recently patented continuous trace gas preconcentrator (CTGP) provides a unique approach for enhancing the trace gas concentration, without stopping the flow. In this study, a performance model is developed for the CTGP, by application of the Poiseuille flow coefficients for long tubes. Based on the Cercignani–Lampis scattering kernel, Sharipov calculated the Poiseuille flow coefficients for various geometries and numerous operating Knudsen number...

Characterization of a Fast Responding Composite Thermal Bimorph Film Actuator Based on Carbon Nanotube Sheets

As the extraordinary thermal, electrical, and mechanical properties of carbon nanotubes (CNTs) have become better understood, they have found their way into a wide range of engineering applications. Used in conjunction with fiber-reinforced composite materials, CNTs provide enhanced thermal conductivity, interlaminar strength, and ballistic resistance of laminar composite materials. However, the direct application of the macro form of CNT sheet as a heating element for use in a thermal actuator has not been reported. In the present study, CNT sheets are used as a flexible, efficient, and fast-responding heating element that induces transverse motion in a multilayered functional polymer composite based on thermal expansion mismatch between layers. The CNT heating element is designed to have a specific cross-sectional area to length aspect ratio, giving it a specific resistance and power consumption characteristic. The heating element is bonded to a compliant silicone elastomer substrate and a stiff constraining polyimide thin film, forming a flap-like actuator. The robust design and simple operation of the actuator makes it a potential candidate for control surfaces on micro air vehicles and actuating elements in microscale fluid pumps. The heating response rate of the actuator is measured experimentally using an infrared thermal imager. The temperature change in the thermal actuator is measured as a function of input voltage. The edge deflection of the actuator is also measured as function of the applied voltage. Finally, finite element modeling of the thermal actuator, a parametric study of material selection, and deflection analysis are conducted to better understand the result of these experiments.Copyright

Computational Studies on the Effects of Non-Linear Temperature Functions in Thermal Creep Membranes of Radiantly Driven Knudsen Compressors

Employing rarefied gas phenomenon of thermal creep (also known as thermal transpiration), Knudsen Compressor is a micro/meso-scale gas compressor/pump without moving parts. Driven by a temperature difference, gas molecules moved from the cold side of the thermal creep channel, which has a size less than the molecular mean free path, to the hot side of the channel. To utilize its low thermal conductivity and nanometer range size pores, carbon opacified aerogel membranes, treated as a bundle of thermal creep channels, were used in prior experimental studies of radiantly driven Knudsen Compressors. By absorbing the radiation energy, a temperature gradient will develop inside of a carbon opacified aerogel membrane to drive thermal creep flows. Analytical studies of the radiation energy absorbed by a carbon opacified aerogel membrane were performed and the resulting non-linear temperature distribution function within the carbon opacified aerogel thermal creep membrane was identified previously. This paper presents DSMC (Direct Simulation Monte Carlo) simulation studies that incorporate the previously reported non-linear temperature distribution function to investigate the performance of the radiantly driven Knudsen Compressor with a carbon opacified aerogel membrane. Cases with different connector temperatures for a closed system Knudsen Compressor were studied to observe the maximum pressure differences. Comparison of results indicates that radiantly driven Knudsen Compressor with a carbon opacified aerogel membrane could achieve a larger pressure gradient than what is predicted by the theoretical model reported by Muntz et al.Copyright

In-Plane Thermal Conductivities of CFRP Composites Interleaved With Dissimilar Conductive Media

Four types of carbon fiber reinforced polymer (CFRP) specimens were prepared with prepregs. Three of the four composite specimens have embedded dissimilar material sheets intended to improve in-plane thermal conductivities. The co-cured subsurface enhancements are non-woven carbon nanotube (CNT) sheets, metallic wire mesh, and metallic surface cladding. One dimensional, steady-state, thermal conduction experiments were carried out using a thermocouple array in a well-insulated space between a heat source and a heat sink. Accounting for heat losses and thermal contact resistances, a best-fit curve from an experimental temperature distribution profile is used to calculate thermal conductivities for all composite specimens. Mechanical property degradations due to the added thermal enhancements were measured using in-plane stiffness, flexural strength, and interlaminar shear strength as benchmarks. The thermal conductivity of the CNT sheet enhanced composite improved by 18% with an insignificant decrease in mechanical properties.Copyright

Computational Study on a Novel Micropump Driven by a Built-In Thermal Bimorph Microvalve

Using the rarefied gas dynamic phenomenon of thermal edge flow, a micropump with a built-in thermal bimorph microvalve is proposed to provide pumping needs for systems such as micro fuel cells. A thermal bimorph cantilever used as a microvalve is located within one gas molecular mean free path away from a narrow flow channel, which connects two larger size connectors on either side. The sharp edge of the heated microvalve, whose length is several times of the gas molecular mean free path, can induce flows along its surface and into the narrow channel. Using the DSMC (Direct Simulation Monte Carlo) simulation technique, the thermal edge flow characteristics are studied computationally to determine the feasibility of the proposed design. The result for a closed simulation domains at steady state determined that a pressure ratio of 1.22 can be achieved by the proposed design. The average flow velocities in open simulation domains were found to be closely related to the heater location. This preliminary computational study has proven that with particular parameters, such as the microvalve’s size and location, matching the gas mean free path, the proposed micropump with a built-in microvalve design appears to be viable to drive the thermal edge gas flows and create noticeable pressure difference to serve as a micropump.Copyright